The invention relates to an interferometric measuring arrangement, preferably for use in chromatography, especially a device for detecting changes in refractive indices in transparent, preferably round, capillaries, by means of interferometric methods according to the precharacterising clause of patent claim 1.
Refractive index detectors are employed in liquid chromatography and especially for HPLC, SFC, FIA and CE uses. The detectors employed can generally be used universally, except where the substance to be investigated has exactly the same refractive index as the liquid in which the substance is present in dissolved form. The solution is generally referred to as the medium or the so-called mobile phase. The refractive index detector which underlies the invention and is used for measuring changes in refractive index is based on interferometry, especially for chromatographic techniques using capillaries. The capillary is used as the optical element in the production of the necessary interferogram. The important problems that arise when such detectors are used concern their low degree of sensitivity and their extreme temperature dependence.
When two beams of the same coherent light source pass along different optical paths and then coincide, amplifying and extinguishing interferences result. The optical path length is defined as the physical path length multiplied by the refractive index. The displacement of the interference pattern can accordingly be used to measure changes in the refractive index in the medium when the physical path length is kept constant. In the present case, the interferences are produced by lateral illumination of the capillary with the coherent light of, for example, a helium-neon laser. An individual laser beam that is directed onto a capillary is usually dispersed in a plane perpendicular to the axis of the capillary. The angle of dispersion of the light intensity profile measured relative to the longitudinal axis of the principal ray is characteristic of the dimensions of the capillary and the refractive index of its walls and the medium located therein.
After impinging on the capillary, the light rays follow different paths, depending on the angle at which they impinge on the wall of the capillary and also depending on the refractive index of the medium. Interference patterns then appear between the deflected rays of different pairings of beams which emerge from the capillary in parallel form. The result is complicated superimposition of interference fringe patterns.
An important problem consists in finding and selecting the interference fringes to be evaluated, because not all interference fringes change when the refractive index of the medium in the capillary changes, such as, for example, interference fringes of rays that bypass the inside of the capillary. Owing to the superimposition of the different interference fringe patterns, it is also very difficult to find a suitable interference fringe (for example one belonging exclusively to one interference group) in order to obtain an undistorted, clear output signal.